A technique is known which comprises coating a powder with another substance to impart a new function thereto in order to use the powder in various applications.
For example, magnetic coloring materials for use in color electrophotography, etc., such as magnetic color toners and magnetic color inks, comprise magnetic particles as a base and coating films having various light reflection-absorption edges. The fine powders for use as spherical spacers for liquid-crystal displays or spherical lenses for optical fibers comprise a base particle comprising a transparent material, e.g., glass beads, and a light-transmitting film covering the surface thereof as a protective film. Some of the powders for use in cosmetics also comprise pigment particles whose surface has been coated with a substance which reflects ultraviolet and infrared rays.
As such a functional powder, the inventors previously proposed a powder comprising a base particle comprising a metal having thereon a metal oxide film having a uniform thickness and containing a metal different from the metal constituting the base particle (see Unexamined Published Japanese Patent Application No. 6-228604). According to this technique, a magnetic material, such as a metal (e.g., iron, cobalt, nickel), an alloy thereof, or iron nitride, is used as a base particle and two or more kinds of metal oxide films having different refractive indexes are formed thereon each in a thickness corresponding to one-fourth the wavelength of an incident light. Due to this constitution, a magnetic powder for magnetic toner is obtained which reflects all of the incident light and has a white color. Further forming a colored layer on the powder gives a magnetic color toner.
The inventors further improved the above powder and proposed a powder comprising a base particle and, formed thereon, not a combination of metal oxide films but plural layers of an oxide film and a metal film alternatively (see Unexamined Published Japanese Patent Application No. 7-90310). According to this technique, a multilayer-coated powder having excellent properties when used as a magnetic color toner or the like is obtained.
In recent years, further improvements in functions and a further reduction in particle size are required in the functional powders described above.
For example, in the field of color electrophotography, it is becoming necessary to obtain images having higher resolution and higher contrast. With this trend, magnetic color toners are required not only to have a reduced particle diameter to heighten resolution but also to be colored more vividly.
With respect to such requirements, a base particle can be colored desirably according to the above-described multilayer-coated powder proposed by the inventors, by superposing either oxide films or a combination of an oxide film and a metal film on the surface of a base particle in such a manner that high-refractive-index films are disposed alternately with low-refractive-index films to thereby cause the coated particles to have an absorption peak at a specific wavelength or to conversely show exceedingly enhanced reflection in a specific wavelength range.
When the above multilayer-coated powder is applied to magnetic coloring material powders of the three primary colors, the cyan (blue) and yellow powders undoubtedly have improved vividness. However, in producing a powder colored in magenta, there have been cases where increasing the number of films especially for the purpose of obtaining a more vivid color results in a narrowed absorption bottom width, so that the resultant color as a whole is bluish. In application to a transparent white powder, it is important to reduce the scattering and reflection on the powder surface to thereby heighten transparency. However, the multilayer-coated powder described above is insufficient in this respect and it has been impossible to sufficiently color a transparent white powder.
These problems are thought to be attributable to the fact that in the above-described multilayer-coated powder proposed by the inventors because the film constitution is limited to a combination of metal oxides or a combination of metal oxides and metals, the range of refractive indexes obtainable in the whole multilayered film is limited and the delicate regulation of refractive index cannot be obtained.
Furthermore, the spherical lenses for use as spherical spacers for liquid-crystal displays, spherical lenses for optical fibers, or the like are required to have a high incident-light transmittance (transparency) on one hand and to have a reduced particle size on the other hand. However, the smaller the particle diameter, the more the reflective scattering is apt to occur on the surface and in the inner parts of the particles. Consequently, reduced particle diameters generally tend to result in reduced transparency.
A technique of heightening the purity of a substance constituting spherical lenses has conventionally been employed so as to obtain transparency. However, since there are differences in refractive index between each lens and substances adjacent thereto, interference occurs at these interfaces due to the differences in refractive index to provide new reflection sources. Thus, merely heightening the purity of the constituent substance brings about a limited improvement in the transparency of the spherical lenses.
For use in cosmetics and the like, powders are required to combine the function of effectively reflecting ultraviolet and infrared rays with the function of transmitting light in the visible region so as to enable the color of the pigment itself serving as a base particle to be observed. However, the conventional powders have been insufficient.
The present invention has been achieved in view of the circumstances described above. An object of the present invention is to provide a powder comprising a base particle which themselves has been colored desirably and having a high light transmittance in the visible region.